1. Field of the Invention
The present invention relates to a method for deciding on the order of a signal detection in a mobile communication system, and more particularly to a method for deciding on the order of a signal detection in a mobile communication system that supports an adaptive modulation technology.
2. Description of the Related Art
Typically, unlike a wired channel environment, a wireless channel environment exhibits a low reliability due to multipath interference, shadowing, radio wave attenuation, time-domain noise, interference, etc. These factors obstruct the heightening of a data transmission rate in mobile communications. Many technologies have been developed to overcome these problems. These representative technologies consist in part of an error control coding technology for suppressing a signal distortion and the influence of noise, and an antenna diversity for overcoming a fading phenomenon.
The antenna diversity receives a plurality of signals that suffer an independent fading phenomena and adjusts for the fading phenomena. The antenna diversity may be classified into time diversity, frequency diversity, multipath diversity, space diversity, etc. The time diversity temporarily obtains the diversity by combining a channel coding and interleaving, and the frequency diversity obtains the diversity by the passing signals transmitted with different frequencies through different multipaths. The multipath diversity obtains the diversity by dividing the multipath signals using different fading information. The space diversity obtains the diversity by using independent fading signals using a plurality of antennas in a transmitter, a receiver, or both the transmitter and receiver. The space diversity uses an antenna array.
However, the error control coding technology and the diversity used for the wireless channels cannot comply with the demand for high-rate data services such as Internet connection and multimedia services. For this, the frequency efficiency should be increased. Mobile communication systems having an antenna array have now been researched in order to increase the frequency efficiency.
The antenna array system is a system in which a transmitter/receiver includes multiple antennas and which uses a spatial domain for increasing the frequency efficiency. Since the time domain and spatial domain have already been limited, a higher transmission rate can easily be obtained by using the spatial domain. The antennal array system includes a system called a ‘V-BLAST (Vertical Bell Lab LAyered Space Time)’ or ‘space division multiplexing’ system proposed by Bell Labs. This antenna array system basically corresponds to a MIMO (Multi Input Multi Output) system in which independent information is sent through respective antennas.
In order to extend the channel capacity so that the antenna array system has a high frequency efficiency, correlation coefficients from among channels formed among transmitting antennas and receiving antennas should be small. If the correlation coefficients among the channels are small, respective information transmitted from the respective transmitting antennas pass through the different channels, and thus mobile stations can distinguish between the transmitted information. If the signals sent from the respective transmitting antennas have the different spatial characteristics, they can be distinguished from one another, and this makes it possible to extend the channel capacity. Additionally, the antenna array system is suitable in an environment where many multipath signals having the different spatial characteristics exist. However, in a LOS (Line Of Sight) environment, the channel capacity of the antenna array system is not greatly increased compared to a single transmitting/receiving antenna system. Accordingly, the antenna array system is suitable to the environment where many multipaths are produced due to scattering objects located between the transmitter and the receiver, i.e., to the environment where respective transmitting/receiving antenna channels have small correlation coefficients or have diversity effects.
If the antenna array is used in the transmitter/receiver, the channel capacity is increased. In this case, the channel capacity is determined, based in part on whether the transmitter/receiver obtains the channel information transmitted from the transmitter to the receiver. If both the transmitter and the receiver have received the channel information, the increase of the channel capacity becomes greatest, while if the transmitter/receiver have not received the channel information, the increase of the channel capacity becomes least. If only the receiver receives the channel information, the increase of the channel capacity is in the middle of the two values as described above. In order for the transmitter to determine the channel information, the transmitter can estimate the channel state or feed the information back to the transmitter so that the transmitter can recognize the channel state.
The channel information required in the antenna array system is a channel reaction among the respective transmitting antennas and the respective receiving antennas, and increases in proportion to the number of transmitting/receiving antennas. The antenna array system that includes the multiple transmitting/receiving antennas has an increased channel capacity in proportion to the number of antennas being used in the transmitter/receiver. The antenna array system has the advantage in that it can increase the channel capacity in proportion to the number of transmitting/receiving antennas. However, it also has the disadvantage that in the case in which the channel information should be fed back, the amount of feedback information is increased according the increase of the number of antennas. In order to solve this problem, a method for increasing the channel capacity by reducing the feedback information is required.
As described above, as the conventional receiving method in the SM-MIMO (Spatial Multiplexing Multiple Input Multiple Output) system, an architecture for removing an interference signal using the V-BLAST (Vertical Bell Lab LAyered Space Time) has been proposed. The V-BLAST architecture is an interference removing architecture that increases the performance of the respective antennas' preferential detection and removes channels having large SINRs (Signal to Interference pulse Noise Ratios) using an SIC (Successive Interference Cancellation) in the system having an equal power and equal modulation rate. This architecture is called a “forward ordering detection architecture”. This forward ordering detection architecture is described in detail in P. W. Wolniansky, G. J. Foschini, G. D. Golden and R. A. Valenzuela, “V-BLAST: An Architecture For Achieving Very High Data Rates Over The Rich-Scattering Wireless Channel”, Proc. Int.Symp.Signals, Systems, Electronics, pp. 295-300, October 1998.
An SM-MIMO system employing an AM (Adaptive Modulation) architecture has recently been developed in order to increase the channel capacity by increasing the data transmission rate. In such a system, the existing forward ordering detection architecture is not effective in error probability. This is because since a high-degree modulation rate is used in a channel having a large SINR, a channel having a greater SINR may produce a higher error probability. In the case of performing an optimal bit allocation and power allocation, the detection order is selected in a direction in which the total SINRs are increased by preferentially detecting and removing the channels having small SINRs. This is called a “reverse ordering detection architecture”. The reverse ordering detection architecture is described in detail in Ka-Wai Ng, Roger S. Cheng, and Ross D. Murch, “Iterative Bit & Power Allocation for V-BLAST based OFDM MIMO System in Frequency Selective Fading Channel”, Proc. IEEE WCNC., vol. 1 pp. 271-275, March 2002, and Young-Doo Kim, Inhyoung Kim, Jihoon Choi, Jae-Young Ahn, and Yong H. Lee, “Adaptive Modulation for MIMO Systems with V-BLAST Detection” IEEE VTC spring, Vol 2, pp 1074-1078, April 2003.
First, the forward ordering detection architecture described in Wolniansky will be explained.
The forward ordering detection architecture is an architecture that first selects a sub channel having the maximum SINR at respective steps, and is suitable where the sub channels have equal power and equal modulation rates. However, the forward ordering detection architecture is not suitable where to there are errors in the adaptive modulation rate. Specifically, in the case of using the adaptive modulation architecture, although a higher-degree modulation architecture should be applied to a channel having a greater SINR, the minimum symbol distance becomes shorter as the degree of the adaptive modulation architecture becomes higher, and this causes a higher error probability at the equal SINR. Accordingly, a lower error probability cannot be guaranteed although the SINR becomes higher.
Second, the reverse ordering detection architecture described in Ng and Kim will be explained.
The reverse ordering detection architecture has been proposed in a system in which the respective sub channels have adaptive power and adaptive modulation architectures. In Ng, the error probability is limited, and bits and powers are allocated in a direction in which the total transmission power is minimized in a state in which the total bits have been allocated. In this case, the detection order in which the transmission power become minimum when the bits and powers are allocated with respect to all the detection orders. In Kim, an architecture for maximizing the total effective SINRs and a reverse ordering for obtaining the similar performance with a simpler process with respect to the detection orders of all the possible cases have been proposed. However, the architecture proposed in Kim can improve the performance only if a discrete optimal bit loading that is known as a “Campello's algorithm” is used. The Campello's algorithm allocates the power to the side in which the necessary power is smallest and increases the bits if one more bit is given. Accordingly, the modulation architectures of all the cases can be used, and, in a system in which the feedback of the allocated power is well performed, the smallest average error probability is obtained if the total effective SINRs are large.
However, in the actual system, the kind of the modulation architectures and the amount of feedback information are limited. Additionally, since this architecture has a larger amount of feedback information with respect to the power in comparison to the modulation architecture, it is preferable in practical use to adjust the rate only with the power kept constant. In this case, however, the respective transmitting antennas show similar performances in error probability with respect to the forward ordering suitable to the system in which the respective transmitting antennas have the equal power and the equal modulation rate, and the reverse ordering suitable to the system that performs the optimal bit loading. This is because the conventional technologies described in Wolniansky and Kim decide the detection order on the basis of the effective SINRs, but the actual error probability cannot be decided only by the effective SINRs.
Since the above-described forward and reverse ordering detection architectures take into consideration only the effective SINRs in the case of performing SIC, they are not efficient in minimizing the average error probability decided by the minimum symbol distances according to the modulation rates and the received SINRs.
FIG. 1 illustrates the construction of the SM-MIMO system for the forward ordering architecture and the reverse ordering architecture.
Referring to FIG. 1, the conventional SM-MIMO system adopting the AM scheme detects the signal by sub-streams using the V-BLAST architecture. In this case, an effective SIR and bit loading calculation unit uses the forward ordering or the reverse ordering according to an SIR (Signal to Interference Ratio) of the channel. In order for the effective SIR and bit loading calculation unit to decide the optimal detection order, the architecture that uses the forward ordering and the architecture that uses the reverse ordering are expressed by the equations illustrated in FIG. 1. The architecture using the forward ordering assumes that the EP (Equal Power) and the ER (Equal Rate) are used. The architecture using the reverse ordering assumes that the PA (Power Allocation) and the AR (Adaptive Rate) are used. The effective SIR and bit loading calculation unit transfers the optimal detection order decided by the architecture that uses the forward ordering or the reverse ordering to a V-BLAST unit, and thus the signals are received through a plurality of antennas by the detection order.
In summary, if the equal power and the equal modulation rates are used, the conventional SM-MIMO system adopting the V-BLAST architecture has a simple structure and a small amount of feedback information. If the power allocation and the adaptive rate are used, the conventional SM-MIMO system adopting the V-BLAST architecture approximates a theoretical capacity, but it has a large amount of feedback information with a complicated structure.